Meaning
Quantitative parameter characterizes the dispersion of values that could reasonably be attributed to a measured dimension or property. Determining the measurement uncertainty of a moulded part is a requirement for any iso 17025 accredited laboratory and is essential for reliable quality control. This value accounts for the known errors in the gauging system and the random variations that occur during the inspection process.
Error Sources
Temperature fluctuations in the moulding hall can cause significant changes in the size of a plastic part while it is being measured. Measurement uncertainty also includes the resolution of the calliper or cmm, the repeatability of the operator, and the stability of the fixture holding the part. In high-precision moulding, these factors can often be as large as 20 percent of the total allowable tolerance.
Statistical Analysis
Calculating the final value involves combining the individual components of error using a root-sum-square method. The expanded measurement uncertainty provides a confidence interval, typically 95 percent, within which the true value of the dimension is expected to lie. Documenting this value in a test report allows the customer to understand the reliability of the data provided by the moulder.
Process Capability
Reducing the margin of error in the metrology lab directly improves the calculated capability of the moulding process.
